Managing AC Power Distribution for Small Cells

by Dan Rebeck, Product Line Manager, Transtector Systems

Small cells have become an integral part of cellular networks as they provide high-density coverage and capacity that complement macro cells and are especially important at higher frequencies where their use is essential. However, like their larger counterparts, small cells rely on AC power from the utility grid, so whether you’re a network engineer or installer, it’s important to understand the role of DC power distribution for small cells and how it should be constructed.

For those not assiduously following developments in wireless technology, a small cell (Figure 1) is a low-powered base station that is used to extend wireless coverage and increase capacity in a specific area. Small cells are typically deployed in high-density urban areas, enterprise and industrial campuses, and other locations where traditional macro cells may struggle to provide adequate coverage and capacity. These small cells are often placed on streetlights, buildings, or other structures and are connected to the core network through fiber or wireless backhaul. As they have a range of just a few hundred meters at millimeter-wave frequencies, they are essential for filling in coverage gaps.

Small but Mighty (Challenging)

Small cells present a broad set of challenges for power distribution and protection as there is often limited space for equipment, so fitting power management within a pole-mount configuration, on the outside of a building, or in an indoor closet or service room can be difficult. Ideally, network engineers could standardize a single small cell configuration and deploy it throughout the network, but this is often impossible because equipment may have varying load configurations and power demands may differ from market to market.

AC power distribution is extremely important when installing small cells because a power distribution system must meet the specific power requirements of the small cell, comply with safety standards and regulations, and ensure proper installation and maintenance. In addition, some small cell installations may face power quality issues or grounding, which can be a challenge, given their physical setting.

The need to meet mandatory requirements presents a challenge in itself because there are so many standards that must be considered. They extend beyond the mandatory UL and NEC standards to local codes, covenants, and physical footprint requirements. For example, control panels are covered by UL508A, panelboards are covered by UL 67, the main disconnect by UL 98, circuit breakers by UL 489, and surge protection by UL 1449. Other relevant standards include those within NEC (NFPA 70E, NEC 100, NEC 230, NEC 250, NEC 285), and IEEE Sections C62.41.1, C62.41.2, C62.45, C62.33, and C62.35.

Small Cells have Unique Requirements

While small cells perform the same basic function as macro cells, they present a broad, unique set of challenges, including the following:

• Limited space for equipment, often on poles or in a base. In addition to housing equipment that delivers services (radios, antennas, etc.), fitting power management within a pole-mount configuration, on the outside of a building, or in an indoor closet or service room can be difficult.
• One size does not fit all. Ideally, network engineers can standardize a single small cell configuration and deploy it repeatedly throughout the network. Unfortunately, in most communications networks, this isn’t always the case. Required equipment may have varying load configurations, and power demands may differ from market to market. Simple, flexible solutions are best, but they’re not always possible.
• Power quality and grounding. Some small cell installations may face power quality issues, or grounding may be a specific challenge, given the physical setting.
• Compliance—general and local. Meeting code requirements for small cells often goes beyond UL and NEC standards (which should always be incorporated). Local codes, covenants, and physical footprint requirements may create additional challenges.

AC Power and the Need for Safety

Safety is extremely important when providing AC power to a small cell because, like all electronic equipment, small cells must be installed to ensure the safety of installers and the general public. There are several key safety considerations when providing AC power to small cells, including:

Electrical safety: Small cells must be designed and installed in accordance with all relevant electrical safety codes and regulations. This includes ensuring that all electrical connections are properly made, that all electrical components are correctly rated and protected, and that proper grounding is provided.

Fire safety: Small cells must be designed and installed to minimize the risk of fire. This includes using fire-resistant materials, properly ventilating the equipment, and providing adequate fire suppression systems.

Environmental safety: Small cells must be designed and installed to reduce the possibility of environmental damage. This includes using materials resistant to extreme weather conditions and other environmental hazards and ensuring that the equipment is properly grounded and protected from lightning strikes.

Physical safety: Small cells must minimize the risk of physical injury. This includes ensuring that the equipment is properly secured to prevent it from falling and that all equipment is accessible for maintenance and inspection.

Equipment protection and reliability: Small cells are susceptible to transient overvoltage caused by lightning and grid switching. Implementing a high-capacity surge protector with a low clamping voltage will protect all the equipment at the site from these damaging events, ensuring that they can support the useful life that is intended by the designer and operator.

Power Distribution in Small Spaces

So how does power distribution fit into a small cell site? Looking at a typical small cell, power equipment must accommodate the following:

• Accept AC utility power directly from the utility transformer or meter, requiring a service entrance rating and provisions for a neutral-ground bond to meet code requirements
• A main circuit breaker to provide a single shutoff point for system power, with the option for an external power disconnect
• Incorporate a field-configurable combination of branch circuit breakers that can evolve with the load requirements for system equipment
• Integrate surge protection to safely and reliably protect all downstream system equipment from lightning and utility transients

Other active equipment in the small cell can include radio gear, antennas, or additional equipment that drives wireless capabilities to end users, manages traffic, and performs other functions. But without properly managed AC power, this equipment is subject to downtime.

Because of the variety of small cell applications, only some installations require the same feature and capability set. However, in general terms, there are several power and cabinet-related requirements that broadly apply to small cells, as shown in Table 1.

The Importance of Surge Protection

Surge protectors are extremely important when used in cellular networks that use small cells. Small cells are connected to the core network through fiber or wireless backhaul, and they are often located in areas that are exposed to various environmental factors such as lightning, power surges, and voltage fluctuations. These environmental factors can cause damage to the small cell equipment and disrupt the normal operation of the cellular network, which can result in service outages and costly repairs.

Surge protectors protect small cell equipment from voltage spikes and surges by diverting the excess voltage to a grounding conductor (Figure 2). They are designed to limit the amount of voltage that can reach the equipment and can help to protect the small cell equipment from damage, thus reducing the risk of service outages and costly repairs.

It is important to note that not all surge protectors are created equal, and it’s essential to use surge protectors that are specifically designed for use in small cell networks. These surge protectors should be tested and certified to meet relevant safety standards and regulations, and they should be rated for the specific voltage and current requirements of the small cell equipment.

In addition, regular maintenance and testing of surge protectors are also essential to ensure that they are operating correctly and effectively. Any issues or malfunctions should be addressed immediately to minimize the risk of failure and to ensure that the small cell network remains reliable.

In summary, surge protectors are extremely important when used in cellular networks that use small cells. They play a vital role in protecting the small cell equipment from damage caused by voltage spikes and surges, thus reducing the risk of service outages and costly repairs and ensuring the reliability and robustness of the cellular network.

Transtector’s Approach

Transtector addresses all these requirements with Small Cell AC Power Distribution Cabinets (SC-2MMA9 Series). These field-configurable systems are service entrance rated, with 120 VAC or 120/240 VAC input voltages, a main disconnect, an external shunt trip button for an emergency power shutoff, and configurable power management equipment inside the cabinet to meet the demands of small cells.

Many small cell applications utilize two power boxes, one for electrical disconnect and one for distribution. However, these functions can be combined in a single solution to streamline installation and minimize risk. This design also eliminates the cost and space of additional disconnect equipment; the combination cabinet meets all key industry standards.

For example, the Transtector cabinet incorporates a reconfigurable branch breaker system. This allows technicians to easily install or replace a circuit breaker using only a screwdriver if requirements change, which is much less complex than cumbersome DIN rail configurations. Expansion, equipment evolution, or other changes are simple, and installation requires no contact with or removal of the power bus that can sometimes void UL compliance.

In addition, the cabinets support a broad range of circuit breaker values, with 1 A to 25 A options available. This enables safe power distribution to a comprehensive variety of equipment types and the flexibility to meet most requirements. In many cases, radio manufacturers mandate specific breaker amperages in the range of 1 to 10 A to maintain the factory warranty.

Typical field configurable QO style breakers offer 15 A as the smallest amperage, which does not support OEM requirements in this example. The Transtector SC-2MMA9 series cabinet provides four breaker amperages in the 1 to 10 A range to right-size a breaker for each radio.

Field-configurable branch breakers also enable staged implementation through the site’s lifecycle. Users need only pay for breakers required at site commissioning rather than additional breakers not required or defined. The simple access design minimizes the time needed to support future colocation installations and equipment configuration changes, resulting in the lowest total cost of ownership through the complete life cycle of each site.

Each cabinet features integrated surge protection devices (SPDs), specifically I2R 75K Series SPDs. These UL 1449 4th Edition (Types 1 and 2) rated components feature 75 kA maximum 8/20µs lightning surge protection. A short circuit current rating of 200 kA provides more than ample protection for small cells. In addition to UL, each SPD meets key IEEE and NEC standards for long-term reliability and minimized degradation.

Additional Important Factors

It’s difficult to overstate the significance of a service entrance rating in small cell power distribution applications. Put simply, the cabinet must meet UL requirements for safe acceptance of power from the utility, with a single disconnect in case of emergency. Without this, safety is at risk. Integrating this capability into the distribution

Cabinet can save space and reduce costs.

Transtector cabinets can also feature an exterior emergency power shutoff button if required. The service entrance rating also contributes to the significance of grounding. Without proper neutral and ground bonding, safety can easily be compromised, but these small cell AC power distribution cabinets integrate these capabilities into the configured solution.

Finally, the cabinet’s compact form factor is essential as small cell installations are inherently size-constricted. Transtector engineered each cabinet to fit inside equipment poles (or bases) when concealment is required. The standard 9-in. width provides flexibility for physical location—whether on a streetlamp pole, a utility pole, hidden on a building site, or elsewhere. The dynamic design enables a scalable, repeatable deployment process to increase efficiency and minimize the potential for error.

Sample Configuration

Transtector recently developed a cabinet configuration deployed in the Chicago metropolitan area. The construction engineer required a compact solution that could be inconspicuously mounted externally on an existing pole and be used to directly receive utility power without the need for a separate disconnect box.

The AC power requirements for these sites included 10 pieces of equipment that operate at 120 VAC as well as a power-hungry rectifier designed to operate much more efficiently at 240 VAC. Surge protection was also specified to protect and support the reliability of each installation as well as to follow the recommendations of the communications equipment suppliers.

Leveraging the flexibility of the Transtector SC-2MMA9 series cabinet, engineers created a service-entrance-rated configuration (Figure 3) with the following specifications:

• 100A Main circuit breaker
• Ten 7 A, 10 A, 15 A and 20 A 120 VAC branch breakers
• 20A, 240 VAC two-pole breaker
• Integral 120/240Vac split-phase SPD to protect all power feeds

This configuration still leaves two available branch breaker positions open for field-configurable future expansion in the future.

Conclusion

Small cells are essential components in modern cellular networks, providing high-density coverage and capacity, improved network performance, cost-effectiveness, flexibility, and better coverage in hard-to-reach areas, and they require innovative solutions for power distribution and surge protection. To ensure you’re on the right track as it concerns small cell power distribution, remember to consider these questions:

• Is the current power architecture meeting code? Are all industry standards met?
• Is the power cabinet, specifically the branch breakers—truly configurable? Can future changes realistically be met?
• Does the power cabinet provide the flexibility to meet load requirements for a variety of equipment?
• How reliable is surge protection? Does it meet a sufficient rating standard?

All of this requires a bit of work, but it’s far less than you’ll encounter later when you find you haven’t checked all the boxes.

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